Cationic clays are widely distributed in nature and find extensive use in various chemical processes as catalysts and adsorbents. In contrast, anionic clays occur less widely in nature and find only limited use in chemical processes. The interest in anionic clays recently has soared, perhaps in pan because of the recognition that their properties are so different and distinct from more common clays as to pique one's scientific curiosity regarding their potential catalytic properties.
Among the anionic clays hydrotalcite is the best known and has the formula Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3.4H.sub.2 O, with manasseite, a polymorph, having the same formula. Pyroaurite and sjogrenite are polymorphs of formula Mg.sub.6 Fe.sub.2 (OH).sub.16 CO.sub.3.4H.sub.2 O. Among other naturally occurring clays having the formula X.sub.6.sup.2+ Y.sub.2.sup.3+ (OH).sub.16 CO.sub.3.4H.sub.2 O may be mentioned stichtite and barbertonite, polymorphs with X=Mg and Y=Cr, takovite (X=Ni and Y=Al), reevesite (X=Ni and Y=Fe) and desautelsite (X=Mg and Y=Mn).
Although the foregoing formula is that of the "ideal" structure for hydrotalcite and its related minerals, it has been known for some time that analogous anionic materials more generally have the formula [M(II).sub.1-x M(III).sub.x (OH).sub.2 ].sup.x+ (A.sup.n-.sub.x/n).mH.sub.2 O (F. Cavani et al., Catalysis Today, 11, 173-301 (1991), at page 179) where x=0.25, n=2, m=4, and a=CO.sub.3 corresponds to the foregoing cases. Using M(II)=Mg, M(III)=Al, and A=CO.sub.3.sup.= as an example, x may vary over a rather broad range of about 0.1 to 0.34, corresponding to a magnesium/aluminum ratio as high as 9 and as low as about 2. We shall refer to materials deviating from the formula for the "ideal" as synthetic hydrotalcites.
In U.S. Pat. Nos. 3,879,523, 3,879,525, and 3,796,792 Miyata et al. describe "composite metal hydroxides having a layer [sic] crystal structure and to a process for the preparation of the same" of formula EQU M.sub.x.sup.2+ M.sub.y.sup.3+ (OH).sub.2x+3y-2z (A.sup.2-).sub.z.aH.sub.2 O
where the divalent metal could be copper, beryllium, calcium, strontium, barium, zinc, cadmium, tin, lead, manganese, magnesium, and metals of Group VIII, and the trivalent metal could be metals of Group III, titanium, metals of Group V, chromium, manganese, metals of Group VIII, the rare earths and actinides. For Fe.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3.4H.sub.2 O the patentees noted that both differential thermal analysis and thermogravimetric analysis showed a first endotherm at 230.degree. C. corresponding to the loss of 4H.sub.2 O, with another at 370.degree. C. corresponding to a loss of 8H.sub.2 O and CO.sub.2. Calcining of their materials afforded a spinel structure, and the patentees noted that dehydration was reversible, with the material rehydrating to the layered double hydroxide structure. Miyata et al. also exemplified several ternary systems and in the latter two of the patents cited above the patentees specifically described cases where M.sub.x was magnesium.
In UK 1,380,949 and 1.380,950 the patentees described as carriers for Ziegler-type catalysts materials obtained by heating at 110.degree.-600.degree. C. layered double hydroxides of formula EQU M.sub.x.sup.2+ M.sub.y.sup.3+ (OH).sub.o (CO.sub.3).sub.p.qH.sub.2 O
where x was an integer from 2 to 8, y was an integer from 2 to 4, o was an integer from 12 to 18, and p was 1 or 2, followed by chlorination to a chlorine content of 20-70%. The divalent metal could be beryllium, magnesium, calcium, strontium, barium, manganese, iron, cobalt, nickel, zinc, and cadmium in any combination and the trivalent metal was chromium, iron, aluminum, or gallium in any combination. In UK 1,342,020 the patentees described a subgroup where the divalent metal was manganese, nickel, cobalt, copper, zinc or iron and the trivalent metal was aluminum, chromium, or iron and note that whereas the materials made by calcining and reducing the foregoing layered double hydroxides are valuable hydrogenation catalysts, those treated only by calcination were highly efficient dehydration catalysts. Thus, like Miyata et al. the patentees here teach rehydration of calcined material.
Later Miyata et at. (U.S. Pat. No. 4,642,193) teach that layered double hydroxides, and in particular as to those layered double hydroxides where the divalent metal is magnesium, nickel or zinc and the trivalent metal is selected from aluminum, iron, or chromium, calcination at temperatures up to 900.degree. C. produces a metal oxide solid solution (MOSS)--i.e., a homogeneous material where the trivalent metal dissolves in a divalent metal oxide to form a solid solution--which is again convened to the layered double hydroxide in the presence of water. In fact the patentees use the metal oxide solid solution to purify cooling water and specifically teach their rehydration. In U.S. Pat. No. 4,562,295 Miyata et al. teach binary MOSSs for purifying cyclohexanone containing byproduct organic acids where the divalent metal is magnesium, calcium, zinc, cobalt, nickel, or copper and the trivalent metal is aluminum, iron, chromium, nickel, cobalt, or manganese. Finally, Broecker et al. in U.S. Pat. No. 3,990,866 calcined dried Ni.sub.5 MgAl.sub.2 (OH).sub.16 CO.sub.3.4H.sub.2 O at 350.degree.-550.degree. C. and reduced the calcined material to one having zerovalent nickel which was subsequently used as a catalyst in the steam cracking of hydrocarbons.
In this application we describe some ternary metal oxide solid solution systems which have some totally unexpected and extremely useful properties. Whereas the totality of the prior art teaches that the MOSSs are rehydratable to the corresponding layered double hydroxide, the ternary MOSSs of our invention are quite resistant to rehydration. This means they can be used in aqueous or partly aqueous systems for extended periods of time without any physicochemical changes in the system where they are employed, which is important in, for example, circumstances where metal oxide solid solutions exhibit catalytic or adsorbent properties different from, or absent in, layered double hydroxides, or where the metal oxide solid solution is an effective carder (support) for catalytically active species incompatible with layered double hydroxides, or where the catalytically active species manifests different activity when composited on the layered double hydroxides than on the MOSS.
Additionally, some ternary MOSSs of our invention show unexpected non-linear basic properties upon introduction of a ternary metal. For example, in a system of formula Z.sub.6 Al.sub.2 O.sub.8 (OH).sub.2 (vide infra) where the divalent ion Y is substituted for the divalent cation Z, EQU Z.sub.6 Al.sub.2 O.sub.8 (OH).sub.2 .fwdarw.Z.sub.(6-x) Y.sub.x Al.sub.2 O.sub.8 (OH).sub.2 .fwdarw.Y.sub.6 Al.sub.2 O.sub.8 (OH).sub.2
and Y.sub.6 Al.sub.2 O.sub.8 (OH).sub.2 is substantially more basic than Z.sub.6 Al.sub.2 O.sub.8 (OH).sub.2 one would expect the basicity to change linearly with increasing amounts of Y. In fact, in some of these systems we have observed that the basicity changes most rapidly with the introduction of relatively small amounts of Y. This affords one the opportunity of effecting significant changes in basicity while effecting only minor changes in other MOSS characteristics.